The present invention relates to a fuel injection system for an internal combustion engine, and more particularly it relates to a fuel injection system in which an amount of fuel supplied to an internal combustion engine is electrically controlled.
In a fuel metering system for an internal combustion engine, it is well known that an electromagnetic valve which opens to inject a pressurized fuel is disposed in each of a group of intake manifolds communicating with respective cylinders of the engine and that a pressure regulator is provided to regulate a pressure of the pressurized fuel at a constant value. The electromagnetic valve is activated in synchronized relation with the rotation of a crankshaft of the engine and an opening interval of time .tau.(.tau.=K'.multidot.Qa/N where K': constant) is determined by an electric control circuit in response to a rotation speed N of the crankshaft and an amount of air Qa sucked through an intake pipe communicating with the intake manifolds. From cost saving spirits, this conventional fuel injection system is not desirable, since as many electromagnetic valves as the number of cylinders are necessitated.
To provide a fuel injection system which is low in manufacturing cost, it is suggested that the electromagnetic valve is disposed singly in the intake pipe and that the electromagnetic valve is activated by the electric control circuit at least as many times as the number of suction strokes of the engine. According to this suggestion, an allowable maximum opening interval of time .tau..sub.M of the electromagnetic valve under the maximum rotation speed (6,000 r.p.m.) of the crankshaft is determined as follows on an assumption that the engine is in a four-cylinder four stroke type. EQU .tau..sub.M =1/((6,000/60).multidot.(1/2).multidot.4)=0.005 (sec.)
Since the maximum opening interval of time of the electromagnetic valve is required in general to be four times longer than the minimum opening interval of time of the electromagnetic valve,. the minimum opening interval of time .tau..sub.m at the maximum rotation speed (6,000 r.p.m.) is limited as follows. EQU .tau..sub.m =.tau..sub.M /4=0.00125 (sec.)
Since the electromagnetic valve has a response delay time, generally some 0.001 (sec.), from closing to opening, the response delay time is not negligible relative to the minimum opening interval of time .tau..sub.m. This means that a precise fuel metering cannot be performed by the electromagnetic valve.